1. Field of the Invention
This invention relates generally to reading and writing data on a magnetic storage medium and in particular to reading and writing data at a predetermined location on a magnetic storage medium by using servo information embedded within the data.
2. Prior Art
Typically, a disk drive contains one or more circular planar disks that are coated on each side with a magnetic medium. The disk or disks are mounted on a spindle that extends through the center of each disk so that the disks may be rotated at a predetermined speed, usually about 3600 rpm. Usually, one read/write head is associated with each side of the disk that is coated with a magnetic medium. The read/write head flies a small distance above the disk surface as the disk rotates. The read/write head, in response to signals from electronics associated with the disk drive, writes data at a predetermined location in the magnetic medium. Similarly, the read/write head, in response to other signals from the disk drive electronics, reads the data stored at a predetermined location.
The configuration of the data on the magnetic surface is instrumental in the operation of the disk drive. Data are recorded by the read/write head in concentric circular tracks on the disk. Corresponding tracks on different disk surfaces are cylindrically aligned.
Typically, each track is segmented into one or more parts that are referred to as sectors. Thus, the disk drive must move the read/write head radially across the disk surface to locate the track for reading or writing data and then must follow that track circumferentially until the desired sector passes under the read/write head. Hence, the read/write head is positioned at a predetermined radial and circumferential position over the disk surface.
In a disk drive, each read/write head is usually affixed by an arm to an actuator and the actuator is moved so that the read/write head is moved radially to a specified track. This operation is referred to as a track seek, or sometimes just a seek. In an open-loop disk drive, a stepper motor is used to move the actuator while in a closed-loop disk drive a servo system is used to move the actuator.
Many different servo systems have been developed for use in hard disk drives. In a servo system, the read/write head reads a servo pattern contained in a servo field to determine the radial and circumferential position of the read/write head relative to the disk. The information that is read is provided to the disk drive control loop electronics which in turn repositions the read/write head as necessary based on that information. Good servo control is essential for reliable storage and retrieval of data on rotating memory storage devices such as hard disk drives.
The servo pattern within the servo field is the key to good servo control. The servo pattern must provide the control loop electronics with an accurate read/write head position, both radially and circumferentially relative to the disk. Typically, for radial positioning, two sub-fields, i.e., a cylinder address sub-field and a position sub-field, within the servo field have been used. Usually, these sub-fields are positioned adjacent to one another in the servo field.
The cylinder address sub-field contains a Gray code pattern that (i) identifies the track containing the servo field and (ii) is a coarse radial position indicator. The Gray code track number pattern is a set of magnetic dibits that contain the track address. As is known to those skilled in the art, the track addresses are addresses that are encoded using a Gray code sequence so that any decoding uncertainty is limited to plus or minus one half track. With the Gray code, only one bit in the track address changes from track to track.
The position sub-field is usually adjacent to the cylinder address sub-field and consists of a magnetic pattern that generates a series of pulses. The disk drive electronics detects the peak of each of the series of pulses. When the read/write head is radially centered over the track, the peak amplitude of pulses adjacent to and on both sides of the center of the track are equal. The difference between the peak amplitude of adjacent pulses corresponds directly to the amount the read/write head is off from the center of the track. Hence, the position sub-field is a fine radial position locator for the radial positioning of the read/write head.
The circumferential positioning information in the servo field is also broken into course and fine position information. Typically, an index sub-field, that consists of one or more bits, is used as a course locator of a point along the circumference of a track. The index bits usually are used to identify only one sector on a track that may contain for example 72 sectors.
A more precise indicator of circumferential position if provided by the sector mark sub-field within the servo field, which is used to precisely identify the location of each sector in a track. A sector mark bit in the sector mark sub-field is used to generate a signal at a precise location. To locate a particular circumferential position, the number of sector mark signals that occur after an index signal are counted. Since the sector mark signal is a precise circumferential positioning mark, it is used by the disk drive spin control hardware as a tachometer signal to control the speed of the disk relative to the read/write head.
One type of servo system is an embedded servo system where a servo field identifying the circumferential and radial data location is placed in front of each data sector in a track. One example of an embedded servo pattern is given in U.S. Pat. No. 4,823,212 issued to Knowles et al. on Apr. 18, 1989 where each track is divided into an equal number of sectors. Each sector includes a section of servo code, referred to as a servo field 100, at the beginning of the sector. Servo field 100 demonstrates each of the general features discussed above.
Each servo field 100 is the same length and includes, starting at its leading edge, a write splice sub-field 101, an automatic gain control (AGC) sub-field 102, a sector mark sub-field 103, an index sector identifier 104, a defect bit 105, a Gray code track number sub-field 106, and a track position sub-field 107 followed by another write splice sub-field. Servo field 100 is preceded and followed by data regions 110 and 111, respectively. As explained more completely below, AGC sub-field 102 is actually divided into two parts. The first part is a write-to-read transition zone and the second part provides the actual AGC data.
FIG. 1B is a flat view of the magnetic dibits in one servo field in tracks 3 to 6 of the disk. The other servo fields and data fields have the same general structure as illustrated by the block diagram of FIG. 1A. FIG. 1C is the signal pattern generated when the information in track 3 is read.
Write splice sub-fields are used to compensate for disk rotational speed variations so that servo field 100 is not overwritten by data. The AGC portion of automatic gain control sub-field 102 is used to normalize the signals from the read/write head so that subsequent servo information is properly detected and processed. Sector mark sub-field 103 was described above. Index sector sub-field 104 was also described above. Defect bit 105 is used to indicate that the data region associated with servo field 100 is defective. Finally, track position sub-field 107 is used to generate signals that are used for track following, as described above. The relative position of the various sub-fields within servo field 100 are typical for an embedded servo system.
Several different approaches have been used in the track position sub-field of the servo field to encode information that results in accurate track following. For examples of track positioning techniques, see U.S. Pat. No. 4,823,212 issued to Knowles et al. on Apr. 18, 1989; U.S. Pat. No. 4,530,019 issued to Penniman on Jul. 16, 1985; U.S. Pat. No. 4,424,543 issued to Lewis et al. on Jan. 3, 1984; and U.S. Pat. No. 4,669,004 issued to Moon et al. on May 26, 1987, which are incorporated herein by reference in their entirety.
To obtain the position information contained in a servo field 100, the disk drive read channel must be prepared so that the information in servo field 100 is accurately detected and read. Not only must the disk drive read channel be prepared, but also the pattern read must be synchronized with the disk drive detection hardware so that the information in servo field 100 can be read and stored.
There are two factors in preparing the read channel. First, adequate time is required for the automatic gain control circuitry to adjust for gain differences between the gain in the prior operation and the gain in servo field 100. Second, if servo field 100 follows a write operation, a write-to-read settling time is required for the disk drive electronics to shift the mode of operation.
Another important factor in AGC control is the amount of AGC data in the servo field. For example, AGC field 102 includes 63 bits of AGC data where a bit is defined as the minimum period of time between adjacent peaks of opposite polarity in the signal trace (FIG. 1C). However, only thirty-six of these bits are actually used for AGC control. (See FIG. 6B of U.S. Pat. No. 4,823,212 of Knowles and Kier issued on Apr. 18, 1989.) The other twenty-seven bits are used for write-to-read settling and are not used for AGC.
In low power computers, the power to the disk drive read channel is frequently turned off over the data region of each sector when the disk drive is idle. When the read channel is turned off, the AGC level is lost. Typically, thirty-six bits of AGC data are insufficient to reestablish an accurate AGC level. Thus, while servo pattern 100 is adequate for larger disk drives that have other than a battery power supply, the pattern will not provide reliable AGC levels for battery powered disk drives that power down the read channel.
The embedded servco patterns are written on the same disk real estate that is used for data storage. Consequently, increasing the AGC field to permit reduced power operation limits the amount of data that can be stored on the disk. While this problem is widely recognized, the accuracy required for radial and circumferential positioning has required that such overhead be absorbed.